Free piston motor-compressors



May 22, 1956 R. HUBER 2,746,670

FREE PISTON MOTOR-COMPRESSORS Filed May 11 1951 W7/% A TTORNE6 UnitedStates Patent 2,746,670 FREE PISTON MOTOR-COMPRESSORS ApplicationMay 11,1951, Serial No. 225,691 Claims priority, applicationFrance May 13, 19504 Claims. (Cl. 230-1-56) The present invention relates to free pistonmotor compressors in which the movement of the movable system or systemsin one direction (outward stroke) is produced by the combustion of fuelin the power cylinder of the motor compressor, whereas the movement intheopposed direction (inward stroke) is produced by the energy of areturn energy accumulator which has stored up for this purpose a portionof theenergy supplied by the combustion of fuelv during the precedingoutward stroke.

in the present specification, the term free piston motor compressordesignates a machine the compressor portion of which discharges most ofthe air it compresses to the outside directly, that is to say withoutpreliminary passage through the power cylinder of the machine.

The object of my invention is toprovide a motor compressor of this kindwhich is better adapted to meet the requirements of practice than thoseexisting up to the present time.

A preferred embodiment of my invention will :be .hereinafter describedwith reference to the accompanying drawing, given merely by way ofexample and in which:

Fig. 1 diagrammatically shows a free piston motor. generator madeaccording to my invention.

Fig. 2 diagrammatically :shows differentpositions of a regulating devicebelonging to this motor-generator.

Fig. 3 is a diagram showing diagrammatically, in accordance withvariations of the discharge pressure of the motor compressor, the meanpressures existing in the return energy accumulator.

The motor compressor proper may-be of any suitable construction, such asthat shown by Fig. l. Sucha motor compressor includes a power. cylinder1 provided, in the central part thereof, with at least one, fuelinjector; 2 and in which operate, in opposed directions, two powerpistons 3 and 4 which control respectively the inlet portv5 and theoutlet port 6 of said power cylinder. The power portion of the motorcompressor, constituted by said elements 1 to 6, works on the two strokediesel cycle.

Power piston 3 is connected with a low pressure compressor piston '7working in a low pressure compressor cylinder 8 provided with a suctionvalve 9 and a discharge valve 10.

On the other hand, powerpiston4 is rigid with a high pressure compressorpiston 11 which works ina high pressure compressor cylinder 12 provided.with an inlet valve 13 and a delivery valve 14. The space 15 into whichis delivered the .air discharged from the low pressure compressorcylinder communicates with the inlet of the high pressure cylinder 12through a conduit 16. The air discharged from the high pressnrecylinder-12 is fed to a reservoir 17, from which it canflowthrougha conduit 18to the place where it is to beused.

The two movable system-s3, 7 andA, 11 are connected together byasynchronizingmechanism of a known type (not shown), constituted forinstance by Connecting rods and cranks, or by pinions andrackaandwhichvisintended to give these systems, at any time, respectivepositions symmetrical with reference to the center of the power cylinder1.

In such a motor compressor, the outward stroke is produced by thecombustion of fuelinjected into the power cylinder in the space betweenthe two power pistons 3 and 4, when they are close to their inner deadcenters. During this outward stroke, the low pressure compressor piston7 compresses and delivers through discharge valves 10 the air present inlow pressure cylinder 8, while the high pressure cylinder subjects theair already compressed in the low pressure cylinder to a second stage ofcompression to discharge it into reservoir 17.

The return or inward strokes of free pistons 3, 7 and i, 11 are ensuredon the one'hand by the compressed air cushions which, at the end of theoutward stroke of these pistons, remain entrapped in the clearancespaces of compressor cylinders 8 and 12 and, on the other hand, by areturn energy pneumatic accumulator constituted by the inside of acylindrical chamber 3a formed in piston 3 and which cooperates with astationary piston 19. The energy stored up in this accumulator depends,on the one hand, upon the length of the outward stroke of power piston 3and, on the other hand, upon the mass of air which constitutes thecushion present in chamber 3a. The energy stored up in the accumulatorincreases, on the one hand, when the mass of air forming the cushionincreases and, on the other hand, when the outward stroke of the movablesystems increases, which compensates, at least partly, for the reductionundergone by the return energy of the air cushions of the dead spaces ofthe compressor cylinders when the outward stroke increases.

if the motor compressor worked always with exactly the same deliverypressure, it would not be necessary to provide a device for regulatingthe return energy of accumulator 3a, 19. However, even in a motorcompressor which normally supplies air always at the same pressure(nominal pressure), some variations in the delivery pressure, due tovariations in the load of the compressor are unavoidable. This is whythe motor compressor must be fitted with a regulating device whichvaries the energy brought into play in the accumulator in accordancewith the variations which mayoccur in the value of the'deliverypressureof the compressor.

It should be noted here that the final pressure of com-' pression in thepower cylinder depends upon the amount of energy stored up in theaccumulator and that this pressure, on the other hand, is one of thefactors which determine the number of oscillations of the movablesystems per unit of time, this number being higher as .the finalpressure of compression in the power cylinder is higher.

Furthermore, it should benoted that the finalpressure of compression inthe power cylinder can vary within rather wide limits.

As above stated when a motor compressor is arranged to work with a givendelivery pressure called nominal pressure, .thisdoes not mean that thedelivery pressure is constantly equal to said nominal value and slightvariations are unavoidable. This is due inv particular to the fact thatthe obtainment of the desired delivery pressure (pressure in reservoir17) is obtained by means of a regulating device, of conventionalconstruction and not shown by the drawing, which controls the fuelinjection (ml) in accordance with the pressure in reservoir 17. Whenthis pressure exceeds the desired value due to the fact that there is noload, the fuel injection is cut oil and the motor compressor ceasesfeeding air to said reservoir 17. But it will be understood that sincethere is no load (i. e. since there .is no outflow from reservoir 17)the slight excess of pressure which has caused the regulating device tocut ofifuelinjection remains, and the pressure in reservoir 17 is thenslightly higher than when the motor compressor is working on full load(i. e. reservoir 17 is delivering compressed air to receiver machines).

In other words, the nominal pressure which is to be achieved at thedelivery of the motor compressor is not a fixed value, but a range ofvalues, with an upper limit (corresponding to no load) and a lower limit(corresponding to full load), the interval between these limits(hereinafter called regulation difierence") being small, for instancesome tenths of an atmosphere.

Starting from these considerations, I arrange, according to myinvention, the regulating device of the return energy accumulator sothat the final pressure of compression in the power cylinder isrelatively high, for instance 60 atmospheres or more, as long as thedelivery pressure is lower than or at most equal to the lower limit ofthe nominal delivery pressure, that is to say the value whichcorresponds to the maximum load of the motor compressor, and that thefinal pressure of compression in the power cylinder decreases quickly assoon as the delivery pressure exceeds said lower limit of its nominalvalue, to come to a minimum value when the delivery pressure reaches theupper limit of its nominal value.

In other words, according to my invention, the return energy regulatingdevice is arranged in such manner as to work in accordance with twodifferent laws one of which determines the operation of this device aslong as the delivery pressure has not yet reached its minimum nominalvalue, whereas the other one determines its operation when the deliverypressure is within the limits of the above mentioned regulationdifference.

I thus obtain, in the period for which the motor compressor is workingon a load lower than the maximum load thereof, a reduction of the numberof oscillations per unit of time, which reduction is the greater as theload of the motor compressor gets nearer to its minimum value, andpossibly to its zero value. This reduction of the number of oscillationsper unit of time gives for the motor compressor the same etfect as areduction of the length of stroke of the movable systems. The reductionof the number of oscillations, for loads lower than the maximum load,makes it possible to reduce the differences between the maximum andminimum strokes of the movable units.

When the return energy accumulator is a pneumatic accumulator, theenergy brought into play in this accumulator is regulated by varying themass of air contained in the accumulator, which mass is, for a givenlength of stroke, proportional to the mean pressure in the accumulator.

It is clear that there are different ways, according to my invention asabove set forth, of devising a motor compressor and the device forregulating its return energy.

A particularly advantageous embodiment of my invention is shown by theappended drawing. According to this construction, the device forregulating the mass of air in accumulator 3a, 19 includes a slide valvedevice 20 connected through conduit 21 with the inside of accumulatorcylinder 3a. Slide valve 20 is adapted to connect, according to itsposition, conduit 21 either with the compressed air. reservoir 17,through a conduit 22 having a throttled portion 23, or with theatmosphere through a tube 24.

Slide valve 20 is made rigid with a piston 25 which cooperates with acylinder 26 the upper chamber of which communicates through a throttledpassage 27 with conduit 21, whereby the mean pressure of the accumulatorair cushion is transmitted to said chamber of cylinder 26 and acts uponthe upper face of piston 25. The other face of this piston is subjectedon the one hand to the action of a compression spring 28 working inopposition to the accumulator mean pressure and on the other hand to theaction of an adjustable pulling spring 29 exerting a force of the samedirection as that of the accumulator mean pressure acting upon the upperface of piston 25. Pulling spring 29 is adjusted in accordance with thedelivery pressure existing in the reservoir 17 of the accumulator.

For this purpose, this delivery pressure is made to act through aconduit 30 upon a manometric box 31 placed in a casing 32 so that itstop end 31a is subjected on the one hand to the action of said deliverypressure and on the other hand to that of a spring 33 which is given apreliminary compression. This manometric box is constituted by a kind ofbellows 31 the lower edge of which is fixed in an airtight fashion tothe cylindrical wall of cylinder 32 in which it is coaxially mounted.The flat top end 31a of said bellows carries, fixed thereto, a rod 31bhinged to the end of a lever 40 to an intermediate point of which ispivotally mounted the rod 41 of a slide valve 34 controlling the supplyof a fluid under pressure fed through conduit 42 to a cylinder 43containing a piston 35 the rod of which is pivotally connected to theend of a lever 36 to the other end of which pulling spring 29 issecured. In order to vary the tensioning of spring 29 in accordance withtwo different laws, this lever 36 bears successively upon two differentfulcrum abutments 37 and 38 each analogous to the knife-edge of abalance. The effect of these two fulcrum abutments upon the operation oflever 36 is shown on a larger scale by Fig. 2. For all positions of thelever between position I, which corresponds to a delivery pressure )0lower than the lower limit 17, of the nominal pressure of the motorcompressor, and position II, which corresponds to this nominal pressurelower limit, lever 36 bears against fulcrum 37, whereby thedisplacements of its right hand end are transmitted with a reduction tothe left hand end, i. e. to spring 29. Consequently, the variations oftension of spring 29 are relatively small as compared with thevariations of the delivery pressure. When the delivery pressure exceedsvalue p1, lever 36 ceases to bear against fulcrum 37 and comes to bearagainst fulcrum 38. From this time on, the displacement of the righthand end of lever 36 is transmitted with an amplification to the lefthand of this lever and therefore to spring 29. Thus, the law accordingto which the tension of spring 29 varies as a function of the deliverypressure is different for the pressures lower than or equal to p1 andfor the pressures ranging from )1 and pm. This last mentioned pressure,which is that corresponding to position III of lever 36 is the upperlimit of the nominal delivery pressure of the motor compressor, which isreached when said compressor is working on no load.

The effect of this change in the law of variation of the energy broughtinto play in the accumulator is illustrated by Fig. 3, which shows thevariation of the mean pressure pm (in ordinates) of the accumulator aircushion as a function of the delivery pressure p in reservoir 17 (inabscissas). This diagram shows that 2111 decreases slowly when thedelivery pressure increases from pc to 121. Preferably, the law ofvariation of pm as a function of p is such that the reduction thereofbalances the increase in the return energy which takes place in the aircushions of the clearance spaces of compressor cylinders 8 and 12 at theends of the outward strokes. Thus, the sum of the return energies actingupon the moving systems remains substantially constant and has, forinstance, a value which gives the final pressure of compression in thepower cylinder a value approximating 60 atmospheres.

Now, when the delivery pressure exceeds value p1, the change in the lawof variation of the tension of spring 29 causes pm to decrease much morequickly, as shown by line a-d, the value of pm at point d of the curvebeing that existing in the accumulator air cushion when the deliverypressure is [7121. This reduction of the mean pressure of the pneumaticaccumulator air cushion causes a substantial reduction of the finalpressure of compression in the power cylinder and consequently areduction of the number of oscillations of the moving systems per unitof time. The reduction of this final pressure of compression may be suchthat for a delivery pressure equal to the upper limit of the nominaldelivery pressure pm, said final pressure is only 30 atmospheres.

ire-

When the same motor compressor must be capable of giving at severaldifferent nominal pressures, the above described regulating device caneasily be adapted to these different nominal pressures. It suffices, forthis purpose, to provide for fulcrum abutment 38 several positions (byinsertion thereof in different holes of the machine frame) each of whichholes corresponds to a given nominal pressure. Fig. 1 shows a position382 of said fulcrum abutment for a nominal delivery pressure lower limitp2, with a nominal delivery pressure upper limit 1722,, and a position383 for a nominal delivery pressure lower limit 113, with a nominalpressure upper limit P39.-

When fulcrum 38 is placed in position 382, fulcrum 37 remains operativefor delivery pressures increasing up to pressure p2 (point b of thecurve) and lever 36 comes to cooperate with fulcrum 382 only when thedelivery pressure exceeds p2, a quick decrease of the mean pressure pmthen taking place along sloped curve b-e. Thus, the final pressure ofcompression in the power cylinder is substantially reduced, for instancefrom 60 to 30 atmospheres.

Finally, when fulcrum 38 is placed in position 383, fulcrum 37 remainsoperative for values of pressure pm down to that corresponding to point0 of the diagram of Fig. 3, for which the minimum nominal deliverypressure lower limit is p3, and when the delivery pressure increasesbeyond this value p3, up to p3a, the mean pressure Pm decreases at aquicker rate, along line cf of Fig. 3.

It will further be seen that the slopes of lines ad, b-e, c-f are thesteeper as the nominal delivery pressure is higher.

In all cases, the quick decrease of the accumulator energy as soon asthe delivery pressure exceeds the nominal value lower limitcorresponding to the maximum power causes a great reduction of thenumber of oscillations of the moving systems per unit of time.

In a general manner, while I have, in the above description, disclosedwhat I deem to be practical and efficient embodiments of my invention,it should be well understood that I do not wish to be limited thereto asthere might be changes made in the arrangement, disposition and form ofthe parts without departing from the principle of the present inventionas comprehended within the scope of the accompanying claims.

What I claim is:

1. In combination, a free piston motor compressor including at least onepower cylinder and one compressor cylinder fixed with respect to eachother, a movable system constituted by a power piston and a compressorpiston rigid with each other and freely movable in said two cylinders,respectively, an energy accumulator having a portion thereof operativelyconnected with said movable system for accumulating energy from saidmovable system during the power stroke of said power piston and givingback said energy thereto during the return stroke thereof, a regulatingdevice operatively connected to said accumulator operative in responseto variations in the delivery pressure of said compressor for varyingthe energy stored up by said accumulator on every cycle of said movablesystem so as to reduce said energy when said delivery pressureincreases, said device being adjustable to have two different laws ofvariation of said energy in response to variation of said deliverypressure, one for which the ratio of energy variation to deliverypressure variation is smaller than for the other, and means 6 responsiveto variations in the dehvery pressure of the compressor for adjustingsaid regulating device to work according to the first of said two lawsas long as said delivery pressure is below the lower limit of the motorcompressor nominal delivery pressure and according to the second of saidtwo laws when said delivery pressure is between the lower and upperlimits of said nominal delivery pressure.

2. In combination, a free piston motor compressor including at least onepower cylinder and one compressor cylinder fixed with respect to eachother, a movable system constituted by a power piston and a compressorpiston rigid with each other and freely movable in said two cylinders,respectively, a pneumatic energy accumulator having a portion thereofoperatively connected with said movable system for accumulating energyfrom said movable system during the power stroke of said power pistonand giving back said energy thereto during the return stroke thereof, aregulating device operatively connected to said accumulator operative inresponse to variations in the delivery pressure of said compressor forvarying the mass of air present in said accumulator on every cycle ofsaid movable system so as to reduce said mass of air when saiddeliverypressure increases, said device being adjustable to have twodifferent laws of variation of said mass of air in response to variationof said delivery pressure, one for which the ratio of mass of airvariation to delivery pressure variation is smaller than for the other,and means responsive to variations in the delivery pressure of thecompressor for adjusting said regulating device to work according to thefirst of said two laws as long as said delivery pressure is below thelower limit of the motor compressor nominal delivery pressure andaccording to the second of said two laws when said delivery pressure isbetween the lower and upper limits of said nominal delivery pressure.

3. A combination according to claim 1 including a part movable inresponse to variations of the compressor delivery pressure, the meansfor operating said regulating device including a lever operativelyconnected at one end to said part and at the other end to saidregulating device, and two fulcrum abutments for cooperating with saidlever, said abutments being positioned to be both in contact with saidlever when the delivery pressure is equal to the lower limit of thedelivery pressure nominal value.

4. A combination according to claim 1 including a part movable inresponse to variations of the compressor delivery pressure, the meansfor operating said regulating device including a lever operativelyconnected at one end to said part and at the other end to saidregulating device, and two fulcrum abutments for cooperating with saidlever, said abutments being positioned to be both in contact with saidlever when the delivery pressure is equal to the lower limit of thedelivery pressure nominal value, one of said abutments being positionedto cause the lever to reduce the movements imparted thereto by said partwhen transmitting it to said regulating device and the other abutmentbeing positioned to amplify these movements.

References Cited in the file of this patent UNITED STATES PATENTS1,726,491 Johnson Aug. 27, 1929 2,434,877 Welsh et al Jan. 20, 1948FOREIGN PATENTS 237,642 Great Britain 1925

